Introduction
- Blown film extrusion is a process that is very popular and used these days in the packaging, agriculture, and medical industries. Just imagine how those plastic bags in which you bring groceries or that stretch wrap that keeps the products safe are made. All these are made through blown film extrusion. But this process is not as easy as it seems by looking at the bags or pouches.
- To make a perfect film, one must understand and know how to use the blown film formula. All these formulas decide how strong the film will be, what its thickness will be, how flexible it will be, and what its life will be. But using these formulas perfectly is not that easy. Because many things are involved in it, from resin selection to machine settings, precision is required in every step. So, let’s start:
Table of Contents
Basics of blown film extrusion
- Blown film extrusion is a process that melts plastic resin or plastic material in granular form and converts it into a thin, flexible film. You will find these thin or thick films everywhere—in shopping bags, food packaging, agricultural films, and many other places. Even though the blown film extrusion process is so common, it is a little technical and typical to understand.
How It Works?
- Resin Melting: First, granules of plastic resin (such as LDPE, LLDPE, or HDPE) are fed into the extruder by dosing. There, heat is applied to the resin, which helps it melt.
- Bubble Formation: In the second step, the melted material is taken out of a circular die, which gives it a shape, which forms a tube or bubble. This bubble is cooled with the help of air and stretched upwards with the help of a nip roller.
- Cooling and Flattening: When the material comes out of the die, it is completely like water, then air is given to it with the help of an air ring, which cools it and flattens it with the help of a collapsing frame.
The blown film formula
Blow-up ratio (BUR)
- Blow-up ratio (BUR) is an important parameter used in blown film extrusion to measure how much the bubble has expanded in the blown film process. BUR can also be used to measure how much the final diameter of the bubble is larger than the die diameter.
- Generally, BUR ranges from 2:1 to 4:1.
Example
- For stretch films, a lower BUR range (3:1 to 4:1) is preferred, while for heavy-duty films, a lower BUR (2:1 to 3:1) is preferred.
BUR ‘s Blown Film Formula
- BUR = (Bubble Diameter) / (Die Diameter)
- BUR= 2× Lay flat width (LFW) / π × d
- BUR= 0.636 × LFW / d
- Bubble Diameter: This is the maximum width of the bubble when bubble flattening and it is fully expanded.
- Die Diameter(d): This is the diameter from where the molten material comes out.
Frost Line Height: What It Is and How to Control It
- Frost line height is the point at which the melted material begins to cool and transform into a solid form. It is called the “frost line” because the flow of air forms a visible line over the bubble, above which the bubble forms.
Why is Frost Line Height Important?
- Film Quality: Frost line height directly affects many properties of the film, such as thickness, strength, and surface finish.
- Cooling Efficiency: If the frost line height is not correct, the film will be uneven, which can cause problems with bubble stability or weak spots.
- Production Speed: If the frost line height is correct, the production rate can be increased because it will receive proper cooling, which reduces the chances of problems like thickness variation.
Frost Line Height Calculation formula used in common blown film
- Frost line height (FLH) is the vertical height from this point to the point where the film has fully solidified. Some empirical formulas and experimental data are used to accurately measure frost line height (FLH).
For a general approximation
- FLH=k×D×BUR
Where:
- FLH = Frost Line Height (mm or inches)
- k = Empirical constant (depends on material & cooling conditions, typically between 1.5 to 3)
- D = Die diameter (mm or inches)
- BUR = Blow-Up Ratio (Bubble Diameter / Die Diameter)
Die Gap-Formula
Where:
- G = Die Gap (mm)
- Tf = Final Film Thickness (mm)
- DR = Draw Ratio (Load Stretching Factor of Extrusion Process)
Draw Ratio Formula:
Where:
- Bw = Bubble Width after Blowing (mm)
- Td = Die Thickness (mm)
- Bd = Die Diameter (mm)
- Tf = Final Film Thickness (mm)
Faq of Blown film Formula
What is the blown film formula?
- The blown film formula is a set of calculations and parameters that control the blown film extrusion process. This formula determines the thickness, strength, clarity, and flexibility of the film. It includes factors such as resin type, blow-up ratio (BUR), frost line height, die gap, and air pressure.
There is a blocking issue in the film, how to solve it?
- Use an antiblock additive.
- Improves cooling efficiency.
- Check if there is overheating.
- If there is overheating, then reduce the heat.
- Use slip agent as per its properties.
Ways to improve film clarity?
- Use high-clarity resins (metallocene PE, LDPE).
- Increase cooling rate (better air ring setup).
- Make correct additive selection; keep low antiblock content.
Sir plz tell me how calculate the frost height formula and draw ratio
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